This invention relates generally to semiconductor technology and more particularly to a method of plasma doping a substrate.
One common modern method of doping uses ion implantation. Ion implantation is conventionally performed using very expensive equipment. The expensive equipment requires high throughput to justify the cost of ownership. Techniques are being developed that offer an alternative to conventional ion implantation. These techniques are directed toward low energy, low dose applications, such as those used to produce source/drain extensions for CMOS transistors. As the dimensions of CMOS transistor structures get smaller, shallower junctions are required to maintain, or improve, overall performance of CMOS transistors. Plasma doping is an emerging technology that addresses the needs for shallow junctions, volume of throughput and cost of ownership.
Plasma doping is a method of doping accomplished by ionizing gas to form plasma, and exposing a substrate to the ions produced such that the ions are implanted into the substrate. In plasma doping technology, a plasma is produced by applying radio-frequency (RF) energy to a gas within a chamber. The plasma is created above the substrate to expose the substrate to ionized doping materials within the plasma. A pulsed negative voltage bias is applied to the substrate to attract the positively charged doping materials, also referred to herein as doping ions. The voltage bias amplitude dictates the implant depth distribution. The pulse width, frequency, partial pressure of the doping gas within the plasma, and implant duration control the dosage. If everything else is fixed, the more doping gas within the plasma chamber the higher the dosage. If only doping gas is present, each negative pulse will deliver a large portion the required dose. Diluting the doping gas with other gases will tend to reduce the dose attributable to individual voltage pulses. This will allow for greater control of the overall dose. Commonly used dilutant gases include, argon and hydrogen.
However, the partial pressure is not fixed. As the doping gas dissociates during the formation the plasma the partial pressure increases. Pressure chambers commonly have a pressure sensor and a pump to regulate the pressure within the plasma chamber while continually removing spent gases and potential contaminants. The increase in pressure caused by the dissociation of the doping gas within the chamber tends to cause the pressure system to respond in attempt to reduce the pressure to a predetermined value. This in turn sets up an oscillating pressure within the chamber, as the pressure increases due to dissociation, and then decreases due to pumping only to increase again. This oscillating pressure reduces control over total dosage. A dilutant gas such as hydrogen, which dissociates into hydrogen ions, will also tend to increase the pressure.
Another problem is that the presence of dilutant gas may damage photoresist. Heavy ions such as argon tend to damage the photoresist due to the high energies associated with their impact. This photoresist damage removes photoresist material, which may then redeposit onto the substrate causing contamination.
It would be advantageous to have a method of plasma doping that would reduce, or eliminate, the effects of oscillating pressure within the plasma chamber.
It would be advantageous to have a method of plasma doping that would provide more accurate control of implant duration.
It would be advantageous to have a method of plasma doping that would not produce contamination from redeposition of photoresist.
Accordingly, a method of plasma doping silicon substrates is provided. A silicon substrate is placed within a plasma chamber on a chuck. A voltage bias is applied to the chuck to bias the substrate. Preferably, the voltage bias will be negative pulses. A gas is introduced into the plasma chamber and energized to ignite a plasma, whereby the substrate is exposed to the plasma. Preferably, the negative pulsed voltage bias will attract positive ions formed within the plasma to the substrate. The plasma is then extinguished and the voltage bias removed from the substrate.
Preferably, the gas will comprise a doping gas such as B2H6, BF3, PH3, and AsH3. The gas will preferably further comprise a monatomic dilutant gas.
In a preferred embodiment of the present method, the substrate will be masked using photoresist to provide covered regions and uncovered regions. The uncovered regions will be exposed to the plasma and doped. To avoid contamination caused by redeposition of photoresist, which is caused by impact damage of ions within the plasma, a light dilutant gas will preferably be used such as neon or helium.